1. Field of the Invention
This invention relates to a light emitting diode with such an enhanced radiation property that can be adapted even to a light emitting element to be fed with large current.
Also, this invention relates to a SMD (surface mount device) type LED that can prevent the disconnection of wire or the peeling of wire from substrate or light emitting element in reflowing and sealing.
Herein, an LED chip itself is referred to as light emitting element, and an entire LED chip mounting device including an optical member such as package resin and lens system is referred to as light emitting diode or LED.
2. Description of the Related Art
FIG. 1 is a cross sectional view showing an example of conventional flip-chip type LED. As shown in FIG. 1, the flip-chip type LED 51 is structured such that two electrodes provided at the bottom surface of a light emitting element 52 are connected through Au bumps 53 to an Au plating circuit pattern 54 formed on an alumina substrate 55, and the Au plating circuit pattern 54 is connected to a pair of thick metal leads 56. From the viewpoint of radiation, it is desirable that the light emitting element 52 is connected through the bumps 53 to the metal leads 56. However, a gap between the leads 56 cannot be reduced to less than the thickness of lead member. So, unavoidably, the two-stage structure shown in FIG. 1 is employed. This structure lacks in radiation property since the path to radiate heat generated from the light emitting element 52 is made of thin member. Therefore, the structure cannot be adapted to a light emitting element having a large amount of heat generation, such as a light emitting element to be fed with large current.
Japanese patent application laid-open No. 9-181394 (hereinafter referred to as prior art 1) discloses a laser diode (LD) with improved radiation property.
Prior art 1 relates to a problem that, when an LD with negative and positive electrodes on the same surface and a step between them is soldered to a submount (heat sink), LD is mounted on the submount with a lean so that a contact area therebetween becomes small to lower the radiation. So, prior art 1 improves the radiation from LD to submount by that the submount is provided with the same step as LD such that LD is mounted on the submount with no lean to increase the contact area between the electrodes and submount.
However, prior art 1 does not teach a further radiation means from the submount to outside and, since the submount has equal size to LD and lacks in radiation property, the temperature of LD in operation will rise.
FIG. 2 is a cross sectional view showing an example of conventional SMD type LED. As shown in FIG. 2, the LED 131 is structured such that a metal (Au or Ag) plating circuit pattern 137 is formed on a glass epoxy substrate 132, a light emitting element 133 is mounted on the circuit pattern 137, and electrodes on the light emitting element 133 are wire-bonded through two wires 134 to insulated regions of the circuit pattern 137. Further, the entire device is sealed with transparent epoxy resin 136 to protect the light emitting element 133 and to serve as a lens.
However, since the glass epoxy substrate 132 has a bad radiation property and there is no path to radiate heat generated from the light emitting element 133 in operation, the temperature of light emitting element 133 will rise so that the efficiency of emission is reduced and the life cycle is deceased. So, another type of LED is suggested that, instead of the printed circuit board 132, a light emitting element is mounted on one of a pair of metal leads and is wire-bonded to the other of the metal leads and the entire device is sealed with transparent epoxy resin. In this type of LED, the problem of radiation can be solved since heat is radiated through the metal lead on which the light emitting element is mounted.
However, since the transparent epoxy resin for sealing the entire device has a coefficient of thermal expansion nearly five times that of metal, when subjected to reflowing at a high temperature (about 200 to 300° C.) in surface mounting, the disconnection of wire or the peeling of wire from substrate or light emitting element may occur due to the difference in coefficient of thermal expansion.
Japanese patent application laid-open No. 11-12025 (hereinafter referred to as prior art 2) discloses an example of conventional SMD type LED.
In prior art 2, a ceramics substrate is used that has a good thermal conductivity, a low coefficient of thermal expansion and an excellent heat resistance. The substrate has terminal electrodes on its ends, a light emitting element is mounted on the center of substrate, two electrodes on the surface of light emitting element are wire-bonded to the terminal electrodes, and the entire device is molded with transparent epoxy resin to offer a package.
Although in prior art 2 there is no problem of radiation property because of the ceramics substrate with a good thermal conductivity, the disconnection of wire or the peeling of wire from substrate or light emitting element may occur in reflowing since the entire device is molded with transparent epoxy resin.
Japanese patent application laid-open No. 11-177129 (hereinafter referred to as prior art 3) discloses another example of conventional SMD type LED.
In prior art 3, in stead of transparent epoxy resin, low melting point (nearly equal to 400° C.) glass is used as sealing material. A light emitting element mounted on a ceramics substrate is directly sealed by the low melting point glass without sandwiching resin therebetween. Thereby, the disconnection of wire or the peeling of wire from substrate or light emitting element in reflowing at high temperature can be prevented.
However, in prior art 3, since the low melting point glass has a viscosity significantly higher than that of transparent thermo-setting resin, such as transparent epoxy resin, before setting even when being melted, the disconnection of wire or the peeling of wire from substrate or light emitting element may occur due to stress to be applied by the high viscosity low melting point glass in sealing therewith.
As described above, prior art 1 lacks in radiation property.
On the other hand, prior arts 2 and 3 have the problem that the disconnection of wire or the peeling of wire from substrate or light emitting element may occur in reflowing or sealing, though there is no problem of radiation.